IFilter
An IFilter is a plugin that allows Microsoft's search engines to index various file formats so that they become searchable. Without an appropriate IFilter, contents of a file cannot be parsed and indexed by the search engine.
They can be obtained as standalone packages or bundled with certain software such as Adobe Reader, LibreOffice, Microsoft Office and OpenOffice.
It also refers to the software interface needed to implement such plugins.
How it works
An IFilter acts as a plug-in for extracting full-text and metadata for search engines. A search engine usually works in two steps:- The search engine goes through a designated place, e.g. a file folder or a database, and indexes all documents or newly modified documents, including the various types documents, in the background and creates internal data to store indexing result.
- A user specifies some keywords they would like to search for, and the search engine answers the query immediately by looking up the indexing result and responding to the user with all the documents that contains the keywords.